1. Field of the Invention
The present invention relates to a polyhydroxycarboxylic acid which is useful as a biologically absorbable material, particularly as a sustained-releasable material, and a production method thereof.
2. Description of the Related Art
Some examples of methods for producing a polyhydroxycarboxylic acid which is usable as a biologically absorbable material, particularly as a sustained-releasable material, are known.
For example, there are a method in which lactic acid and glycolic acid are polycondensed in the presence of a strongly acidic ion exchange resin as a catalyst (Japanese Patent Publication No. 21166/1989), and a method in which lactic acid and glycolic acid are polycondensed in the presence of an inorganic solid acid (Japanese Patent Publication No. 78425/1994). Polyhydroxycarboxylic acids obtained by these polycondensation methods have a fault that they contain low molecular-weight oligomers and monomers in a large amount. The oligomers and monomers cause unusual initial release (initial burst) of a drug when such polyhydroxycarboxylic acids are used as a base material of microcapsule.
On the other hand, as a method for decreasing the amount of low molecular-weight oligomers and monomers contained in the polyhydroxycarboxylic acid, for example, Japanese Patent Nos. 2551756 and 3200706 disclose a method of washing the polymer with water or a mixed solution of water and a water-soluble organic solvent.
However, the method of washing the polymer with water has a problem that water must be removed from the washed polymer as much as possible so as to prevent occurrence of hydrolysis of the polyhydroxycarboxylic acid during long-time storage. Such drying step for removing water from the polymer takes long time. Furthermore, if the organic solvent is used, it is difficult to handle the polymer and to remove the residual organic solvent therefrom because the deposited polymer contains the organic solvent and thereby becomes a viscous rice-cake-like mass.
As described above, with the conventional polymerization methods, it is difficult to obtain a polyhydroxycarboxylic acid which has low contents of low molecular-weight oligomers and monomers and has good releasability of a drug. Furthermore, a method of purifying the polyhydroxycarboxylic acid also has various problems. Accordingly, it is desired to develop a polyhydroxycarboxylic acid which has low contents of low molecular-weight oligomers and monomers, contains no organic solvent and is useful as a biologically absorbable material. Furthermore, it is desired to develop a method which is industrially advantageous for producing the polyhydroxycarboxylic acid.